1. Field of the Invention
The present invention is directed to an X-ray diagnostic apparatus of the type having a solid-state radiation detector.
2. Description of the Prior Art
An X-ray diagnostic apparatus of this type is known e.g. from European Application 0 642 264. In this known apparatus, the X-ray beams produced by a beam transmitter are acquired by a solid-state detector with image points arranged in the form of a matrix. Due to the time behavior of the scintillator and the photodiodes after switching off the X-ray radiation, these detectors, based on amorphous silicon, produce afterglow signals, which lead to what are referred to as "ghost" or "phantom" images. In the current X-ray exposure, called the bright image exposure in the following, a residuum of the previous image or images can thus be seen. This effect is very disturbing, and in the X-ray diagnostic apparatus according to European Application 0 642 264 it is corrected by calculating the phantom image still present in the current bright image from previous bright images, and subtracting it. This method is relatively imprecise, and fails in cases of overdriven points (blooming) in the bright image, since the initial brightness value for the calculation of the decay of the afterglow signals is not known.
British specification 1 489 345 discloses a method for the removal of afterglow effects in which bright and dark images are exposed and are mutually balanced (subtracted) in order to remove the disturbance.
It is an object of the present invention to provide an X-ray diagnostic apparatus of the general type described above that has an improved phantom image characteristic.
The above object is achieved in an X-ray diagnostic apparatus according to the invention having at least one beam transmitter, which emits an X-ray beam that is acquired by at least one solid-state detector with image points arranged in the form of a matrix, and at least one correction unit, which acquires the image signals produced by the solid-state detector, of the current bright image exposure of a subject to be examined. In addition, the correction unit acquires any afterglow signals which may be present from at least one dark image exposure. Only the current afterglow can be seen in these dark images. The correction unit controls the beam transmitter dependent on the presence of afterglow signals and/or dependent on the intensity of the afterglow signals. If necessary, the image signals of the current bright image exposure are corrected in the correction unit. The corrected image signals of the current bright image exposure can then be further processed in a known way (e.g. filtering, hard copy, archiving, feeding into networks, etc.).
The inventive X-ray diagnostic apparatus does not emit an X-ray pulse with a bright image being exposed in every possible time interval; rather, at least one dark image is intermittently exposed. From these dark image exposures, in which only the current afterglow can be seen, the afterglow portion in the bright image exposures can be determined and eliminated.
Due to the inventive measure of determining the afterglow signals from at least one dark image exposure, low-dose exposures (transillumination) after preceding high-dose exposures (bright image exposures that are stored) are also possible without difficulty.